@respawn-app/tool-filter-mcp

tool-filter-mcp

MCP proxy server that filters tools from upstream MCP servers via regex-based deny list.

Maintained: ✅ Yes (Winter 2025-26)

Why use this?

For effective context engineering, we want to minimize useless tokens. Most major agents right now (e.g. Claude Code) do NOT remove tool descriptions from the context. Even though the tool is completely denied and unused, the model will still get its entire description and still try to call the tool (and get error messages). For big MCPs, such as github, supabase, jetbrains IDE, atlassian, this is catastrophic to the context and leads to context pollution by 40-60k of useless tokens. You wanted your agent to be able to see jira ticket descriptions? Please also have these 44 useless tools to edit assignees on confluence pages.

This MCP completely solves the issue without introducing any overhead. This project is fully vibe-coded with claude. Contributions welcome!

Features

• Tool Filtering: Block specific tools using regex patterns
• Tool Discovery: List available tools with multiple output formats (table, JSON, names)
• Header Pass-Through: Add custom HTTP headers for authentication
• Zero Latency: Cached tool list, minimal overhead
• Fail-Fast: Immediate error on connection issues or invalid patterns
• Transparent Proxying: Forwards allowed tool calls to upstream without modification

Installation

# For HTTP/SSE MCP servers
npx @respawn-app/tool-filter-mcp --upstream <url> --deny <patterns>

# For stdio MCP servers
npx @respawn-app/tool-filter-mcp --upstream-stdio --deny <patterns> -- <command> [args...]

Usage

Basic Example (HTTP/SSE)

Filter tools matching .*_file$ pattern from an HTTP MCP server:

npx @respawn-app/tool-filter-mcp \
  --upstream http://localhost:3000/sse \
  --deny ".*_file$"

Basic Example (stdio)

Filter tools from a local stdio MCP server:

npx @respawn-app/tool-filter-mcp \
  --upstream-stdio \
  --deny "dangerous_.*" \
  -- npx my-mcp-server

With Environment Variables (stdio)

Pass environment variables to the upstream stdio server:

npx @respawn-app/tool-filter-mcp \
  --upstream-stdio \
  --env "API_KEY=secret123" \
  --env "DEBUG=true" \
  --deny "admin_.*" \
  -- npx my-mcp-server

Multiple Patterns

Use comma-separated patterns:

npx @respawn-app/tool-filter-mcp \
  --upstream http://localhost:3000/sse \
  --deny "get_file_text,create_new_file,replace_text"

With Authentication Headers

Add custom headers for authentication:

npx @respawn-app/tool-filter-mcp \
  --upstream http://localhost:3000/sse \
  --header "Authorization: Bearer your-token-here" \
  --header "X-API-Key: your-api-key"

Headers support environment variable expansion (if not yet expanded by your app):

npx @respawn-app/tool-filter-mcp \
  --upstream http://localhost:3000/sse \
  --header "Authorization: Bearer $AUTH_TOKEN"

List Available Tools

Discover what tools are available before setting up filters:

# Default table format
npx @respawn-app/tool-filter-mcp \
  --upstream http://localhost:3000/sse \
  --list-tools

# Get comma-separated names for copy-paste
npx @respawn-app/tool-filter-mcp \
  --upstream http://localhost:3000/sse \
  --list-tools --format=names

# Get full JSON with schemas
npx @respawn-app/tool-filter-mcp \
  --upstream http://localhost:3000/sse \
  --list-tools --format=json

# Preview filtered results
npx @respawn-app/tool-filter-mcp \
  --upstream http://localhost:3000/sse \
  --deny ".*_file$" \
  --list-tools --format=names

With Claude Code

HTTP/SSE Upstream Server

Add to your .mcp.json:

{
  "mcpServers": {
    "filtered-http-server": {
      "command": "npx",
      "args": [
        "@respawn-app/tool-filter-mcp",
        "--upstream",
        "http://localhost:3000/sse",
        "--deny",
        "dangerous_tool_1,dangerous_tool_2"
      ],
      "type": "stdio"
    }
  }
}

With authentication headers (supports environment variable expansion):

{
  "mcpServers": {
    "filtered-http-server": {
      "command": "npx",
      "args": [
        "@respawn-app/tool-filter-mcp",
        "--upstream",
        "http://localhost:3000/sse",
        "--header",
        "Authorization: Bearer ${API_TOKEN}",
        "--header",
        "X-Custom-Header: $CUSTOM_VALUE",
        "--deny",
        "sensitive_.*"
      ],
      "type": "stdio"
    }
  }
}

Stdio Upstream Server

{
  "mcpServers": {
    "filtered-zen": {
      "command": "npx",
      "args": [
        "@respawn-app/tool-filter-mcp",
        "--upstream-stdio",
        "--deny",
        "dangerous_.*,sensitive_.*",
        "--",
        "npx",
        "my-mcp-server"
      ],
      "type": "stdio"
    }
  }
}

With multiple arguments to the upstream server:

{
  "mcpServers": {
    "filtered-stdio-server": {
      "command": "npx",
      "args": [
        "@respawn-app/tool-filter-mcp",
        "--upstream-stdio",
        "--deny",
        "admin_.*",
        "--",
        "node",
        "my-mcp-server.js",
        "--config=config.json",
        "--verbose"
      ],
      "type": "stdio"
    }
  }
}

With arguments that start with dashes (like uvx):

{
  "mcpServers": {
    "filtered-uvx-server": {
      "command": "npx",
      "args": [
        "@respawn-app/tool-filter-mcp",
        "--upstream-stdio",
        "--deny",
        "test_.*",
        "--",
        "uvx",
        "--from",
        "git+https://github.com/BeehiveInnovations/zen-mcp-server.git",
        "zen-mcp-server"
      ],
      "type": "stdio"
    }
  }
}

With environment variables for the upstream stdio server:

{
  "mcpServers": {
    "filtered-server-with-env": {
      "command": "npx",
      "args": [
        "@respawn-app/tool-filter-mcp",
        "--upstream-stdio",
        "--env",
        "API_KEY=${MY_API_KEY}",
        "--env",
        "DEBUG=true",
        "--deny",
        "admin_.*",
        "--",
        "npx",
        "my-mcp-server"
      ],
      "type": "stdio"
    }
  }
}

CLI Options

Tool list

Print the list of available tools without starting a proxy server

• --list-tools: List available tools from upstream and exit
• --format <type>: Output format for --list-tools (default: table)
  • table: Human-readable table with descriptions (truncated to 100 chars)
  • json: Full JSON output with complete schemas
  • names: Comma-separated tool names for easy copy-paste

Connection Mode (mutually exclusive)

To start the proxy server, you must specify exactly one of:

• --upstream <url>: Connect to upstream HTTP/SSE MCP server
• --upstream-stdio: Spawn and connect to upstream stdio MCP server
  • Requires command and arguments after -- separator
  • Example: --upstream-stdio -- npx zen-mcp-server

Common Options

• --deny <patterns>: Comma-separated regex patterns for tools to filter

HTTP/SSE Mode Options

Only applicable with --upstream:

• --header <name:value>: Custom HTTP header to pass to upstream server (can be repeated for multiple headers)
  • Format: --header "Header-Name: value"
  • Supports environment variable expansion: $VAR or ${VAR}
  • Example: --header "Authorization: Bearer $TOKEN"

Stdio Mode Options

Only applicable with --upstream-stdio:

• --env <KEY=value>: Environment variable to pass to the upstream stdio server (can be repeated for multiple variables)
  • Format: --env "KEY=value"
  • Example: --env "API_KEY=secret" --env "DEBUG=true"
• After the -- separator, provide the command and all its arguments
  • Everything after -- is passed to the upstream server
  • Supports arguments starting with dashes (like --from, --config, etc.)
  • Example: --upstream-stdio -- uvx --from git+https://... package-name

Requirements

• Node.js >= 20.0.0
• Upstream MCP server with:
  • SSE or Streamable HTTP transport (for --upstream), OR
  • stdio transport (for --upstream-stdio)

License

MIT

Copyright (c) 2026 Respawn LLC